The described invention relates in general to a system and apparatus for mixing viscous substances such as dough, and more specifically to a cooling jacket incorporated into the mixing bowl component of industrial mixers for controlling the temperature of the substance being mixed during the mixing process.
Friction and viscous shear encountered during mixing typically causes a temperature rise in a substance being mixed. This temperature rise becomes more severe as mixing speed increases and can adversely affect production by making the substance sticky and difficult to process. Accordingly, mixers, particularly dough mixers, are most effective when equipped with some type of temperature control means, whereby the temperature of the substance to be mixed may be stabilized at a predetermined level or maintained below a predetermined threshold. For example, bread dough should be mixed at a temperature of about 78-80° F. A known means for controlling the temperature of a substance being mixed is through the use of a refrigeration jacket attached to the mixing bowl component of a mixer. Bowl refrigeration jackets, also referred to as “cooling jackets” usually include multiple coolant channels that are arranged perpendicular to the ends of a mixing bowl, and which are arrayed around the profile of the mixing bowl. Additional coolant channels may be optionally included on the ends of the mixing bowl.
Large commercial scale dough mixers may be manufactured both with and without mixing bowl cooling jackets based primarily on the type and quantity of dough to be mixed. Dough mixers manufactured with bowl cooling jackets are categorized as having either “indirect” or “direct” cooling. An indirect refrigeration system utilizes cold water, glycol, or brine as a cooling fluid. This cooling fluid is first chilled by a compressed refrigerant system separate from the mixer, and is then pumped to the mixer. The cooling fluid then circulates through the mixing bowl's cooling jacket, which is typically comprised of a series of parallel channels fastened directly to the exterior of the mixing bowl. Heat generated during the mixing process is transferred from the dough, through the material of the mixing bowl, and then into the cooling fluid. After the mixing process is complete, the cooling fluid is then piped back to a storage tank for reuse. This principle may be applied to a “direct” refrigeration system, as well. A direct expansion refrigeration system introduces refrigerant directly into the refrigeration jacket of a mixer to remove excess heat from the dough being mixed. This type of cooling system typically includes a compressor, a condenser, an evaporator, and a receiver. The bowl refrigeration jacket serves as the evaporator in this configuration and the types of refrigerants used in this configuration typically include R134a and MP-39.
With regard to the commercial mixing systems, during a typical mixing cycle, doughs or other viscous materials approaching 3000 pounds are mixed at speeds approaching 100 rpm. The movement of the material being mixed within the mixing bowl creates tremendous force, which is directed outward in a direction perpendicular to the orientation of the bowl sheet component of a mixing bowl. As a reaction to these forces, the bowl sheet will repeatedly expand and contract, temporarily altering the profile of the bowl sheet. Known systems utilize heavily reinforced bowl cooling jackets to overcome this bowl sheet deflection. Because cooling jackets are usually rigidly attached to the bowl sheet components of mixers to provide sealed channels for coolant passage, the welded points of attachment between channels of the cooling jacket and the bowl sheet are subjected to cyclical stress concentrations. Over time, these stress concentrations result in fatigue at the attachment points and ultimately in the failure of the cooling jacket. Thus, there is a need for a bowl refrigeration or cooling jacket, the design of which reduces the stress and fatigue experienced at the welded points of attachment between the channels of a cooling jacket and the bowl sheet to which it is attached.